Blowout preventers maintain control of downhole pressure in wells during drilling, and ram-type blowout preventers are used to close and seal around a string of pipe or coiled tubing extending into the well to contain the pressure within the well. Variable bore packers have been designed for ram-type blowout preventers to close and seal around tubular members having different diameters within a limited range of sizes. Variable bore packers are designed to adjust their sealing engagement to the particular size of tubular member passing through the ram-type blowout preventer. Various types of prior art variable bore packers have been utilized.
U.S. Pat. No. 4,229,012 discloses a variable bore packer for a ram-type blowout preventer in which irising inserts, operated like a camera shutter, are embedded in a resilient packer. Each insert includes an upper plate, a lower plate, and a rib fixed between the upper and lower plates. Each of the plates is generally triangular in shape and designed to rotate as it moves inwardly with the resilient packer annulus so that the resilient material is supported when in sealing engagement with the exterior of a tubular member extending through the BOP.
U.S. Pat. No. 5,005,802 discloses a variable bore packer having an upper and lower plate embedded in resilient packer material. A series of upper insert segments are positioned in the packer material below the upper plate and are removable with the packer material as it moves forward during sealing. The insert segments move inward with the packer material in sealing to provide an upper anti-extrusion support for the packer material upon sealing engagement around the exterior of a tubular member extending through the blowout preventer. The insert segments include an inner radius sized to match the outside diameter of the pipe against which it is to seal. The insert segments also include a radial length which is sufficiently long to allow them to move into engagement with a pipe exterior and still provide support for the resilient packer material to avoid its extrusion.
As variable bore packers sealingly engage tubular strings of different sizes, it is important to prevent the extrusion of the resilient packer material between the variable bore packer and the tubular member. Prior art packers continue to be subject to extrusion such that upon closing the variable bore packer around the tubular member, minute gaps continue to exist between the packer and tubular member. Such gaps become an increasing problem as the packer wears and is abraded by its sealing engagement with various tubular members passing through the blowout preventer. At times, a “stripping” operation must be performed to strip the string through the closed rams. This stripping movement can severely wear or abrade the face of the resilient packer material.
The problem of extrusion is enhanced with increased downhole pressure and/or increased temperature. As downhole pressures increase to 15,000 psi, such large downhole pressures exacerbate the problem of extrusion due to the great pressure differential across the packer. Seventy or eighty pressure cycles is a typical life span for ambient temperature packers. In high temperature packers, however, much more wear occurs in one cycle than in an ambient temperature packer. Further, as temperatures increase to high temperatures in the order of 350° F., the viscosity of the resilient packer material decreases causing it to be more fluid and thereby more susceptible to extrusion through the minute gaps between the packer and tubular member.
The variable bore packer of U.S. Pat. No. 4,229,012 does not lend itself to high temperature applications because it does not create a tight seal around the tubular member. The irising inserts cannot conform well to the diameter of the tubular member and leave a plurality of small gaps allowing extrusion by the less viscous packer material.
McWhorter et al., in U.S. Pat. No. 5,005,802, provided a packer for use with a ram-type blowout preventer having the capacity to accommodate a plurality of sizes of tubular members extending through the bore of the blowout preventer. The packer had a resilient body to fit the face recess of the ram and a central recess to receive a tubular member, an upper plate positioned in the upper portion of the resilient body, a lower plate positioned in the lower portion of the resilient body, and a plurality of metal insert segments positioned between the upper surface of the resilient body and the under surface of the upper plate and around the central recess of the resilient body.
In U.S. Pat. No. 5,294,088, McWhorter et al. provided another variable bore packer for a ram-type blowout preventer. The packer included a body of resilient packing material with upper and lower plates embedded in the upper and lower surfaces of the body and upper and lower sets of insert segments disposed adjacent the upper and lower plates. Each of the insert segments includes a pair of insert plates forming an arcuate opening to receive an appropriate sized tubular member and dimensioned to expand and move rearwardly in the resilient packing material upon engagement with a larger diameter tubular member.
However, new tubulars in use in the field include a first section of a first diameter, a second section of a second diameter, and tapered section joining the first and second sections. The prior art packers just described are not well adapted to accommodate this new design of tubulars. Further, known packers suffer from excessive tensile stress when subjected to high pressure differentials. The present invention is directed to solving this problem in the art.